It is known that titanium and zirconium alkoxides and aryloxides may become a solid at ambient temperature and pressure. For example, when freshly distilled, titanium tetraethoxide (Ti(OEt)4) is a colourless liquid in the standard state, that is to say at a standard temperature of 25° C. and a standard pressure of 100 kPa. However, in the standard state, liquid titanium tetraethoxide converts to a solid, crystalline mass over time (J. A. Ibers, “Crystal and Molecular Structure of Titanium (IV) Ethoxide”, Nature 197 (4868): 686, 1963). This is also observed at ambient temperature, which may for example be of from −10 to +25° C., for other titanium alkoxides or aryloxides, such as titanium tetramethoxide, titanium tetra(n-propoxide), titanium tetraisopropoxide and titanium tetraphenoxide.
The foregoing also applies to zirconium alkoxides and aryloxides such as for example Zr(OEt)4 and Zr(OnPr)4 (V. W. Day et al., “Isolation and Structural Characterization of Tetra-n-propyl Zirconate in Hydrocarbon Solution and the Solid State”, Inorganic Chemistry 40 (23): 5738-46, 2001).
Ti(OMe)4, Ti(OPh)4 and Ti(OEt)4 have relatively high melting temperatures of 200-210° C., 154° C. and 54° C., respectively, so that these exist as a solid in the standard state (at 25° C. and 100 kPa). In case the alkyl groups are isopropyl or n-propyl groups, as in Ti(OiPr)4 and Ti(OnPr)4, respectively, which have melting temperatures of 17° C. and 20° C., respectively, a liquid does exist in the standard state. However, when the temperature is reduced to a temperature below these melting temperatures, for example during transport and/or storage, these liquid titanium tetrapropoxides would solidify.
Titanium or zirconium alkoxides or aryloxides may be used as catalysts in a variety of chemical production processes. For example, a titanium or zirconium alkoxide or aryloxide may be used as a catalyst in an aromatic carbonate production process. It is known to produce aromatic carbonates from a dialkyl carbonate and an aryl alcohol. For example, the aromatic carbonate may be a diaryl carbonate, such as diphenyl carbonate, which may be prepared from a dialkyl carbonate and an aryl alcohol. In such processes, the dialkyl carbonate is converted into diaryl carbonate via the following steps. In a first step, transesterification of the dialkyl carbonate with the aryl alcohol takes place to yield alkyl aryl carbonate (also an aromatic carbonate) and alkyl alcohol. In a second step, disproportionation of the alkyl aryl carbonate takes place to yield diaryl carbonate and dialkyl carbonate. Further transesterification of the alkyl aryl carbonate with aryl alcohol yielding diaryl carbonate and alkyl alcohol may also take place.
In a chemical production process wherein one of the above-described titanium or zirconium alkoxides or aryloxides having a relatively high melting temperature is employed as a catalyst, in order to hold the titanium or zirconium alkoxide or aryloxide in a molten state, a relatively high temperature has to be employed, for example higher than 55° C. in case above-mentioned titanium tetraethoxide is used. First of all, this imposes an undesired substantial energy requirement. Furthermore, when titanium or zirconium alkoxide or aryloxide is held in a molten state at such a high temperature for a long period of time, it starts being colored, and if it is used as a starting material for producing aromatic carbonates, the color tone of the obtained product tends to be impaired, and the product cannot be used as an ordinary aromatic carbonate product.
Furthermore, the heat stability of titanium or zirconium alkoxide or aryloxide at said relatively high temperature is relatively low. This for example results in that in a case where any moisture (water) is present, the titanium or zirconium alkoxide or aryloxide may easily react with such water at a high temperature. This is illustrated hereinbelow with reference to titanium alkoxide:Ti(OR)4+4H2O→Ti(OH)4+4ROH (hydrolysis)Ti(OH)4→TiO2.xH2O+(2−x)H2O (condensation)
Thus, said reaction results in the production of titanium dioxide of formula TiO2, also known as titanium(IV) oxide or titania. The presence of such TiO2 is disadvantageous as it has little to no catalytic activity, more especially in the above-mentioned process for preparing aromatic carbonates from a dialkyl carbonate and an aryl alcohol. Therefore, it is desirable to prevent the formation of TiO2, and therefore the loss of valuable Ti metal, as much as possible. Furthermore, TiO2 is a powder that is not soluble in most solvents. Therefore, another disadvantage of TiO2 formation is that this powder covers the surface of any heterogeneous catalyst thereby blocking access to catalyst pores. In summary, said reaction with water may result in a catalyst the activity of which may be far less than the desired activity level.
Furthermore, titanium and zirconium alkoxides can undergo a condensation reaction even in the absence of moisture, especially after a prolonged period of time of heating. This is illustrated hereinbelow with reference to titanium alkoxide:(RO)3Ti—OR+RO—Ti(OR)3→(RO)3Ti—O—Ti(OR)3+R—O—R (condensation)
This condensation reaction is disadvantageous, as the Ti—O—Ti containing compound is less catalytically active but also has a melting temperature which is even higher than that of Ti(OR)4.
In addition, most standard transport vessels for liquid materials are not equipped to maintain a temperature above 70° C. However, safe transport and handling of molten titanium or zirconium alkoxide or aryloxide (e.g. with minimal waste from tank washings) requires maintaining the product at a temperature of preferably about 20° C. above the melting point. Transport of liquid materials at such temperature would require a large amount of energy, and could lead to problems with solidifying material if not properly handled. Only a limited number of vessels are even capable of such proper handling at these temperatures, all with rather small tank sizes.
Thus, in view of the above problems associated with a relatively high temperature to keep titanium or zirconium alkoxide or aryloxide in a molten state, it would be better to transport and store titanium or zirconium alkoxide or aryloxide as a solid at ambient conditions. However, this in turn is also problematic. For example, it is problematic to transport solid titanium or zirconium alkoxide or aryloxide catalyst to a chemical production location and to store it at such location in the solid state. For example, in a case where solid titanium or zirconium alkoxide or aryloxide would be handled as a powder, the powder would easily block for example a piping of an apparatus or dissolution in a solvent may be impaired. Therefore, it would be difficult to continue a continuous operation using solid titanium or zirconium alkoxide or aryloxide for a long time, and the operation would have to be stopped repeatedly for washing apparatus such as a conveyer, thus leading to a considerable production loss.
The handling and transport of solid titanium or zirconium alkoxide or aryloxide have drawbacks common to handling of solids in general. Further, in particular, solid titanium or zirconium alkoxide or aryloxide is sensitive to moisture and will then hydrolyze readily, as described above with reference to Ti(OR)4. Therefore, solid titanium or zirconium alkoxide or aryloxide should be handled and transported under a dry nitrogen environment. Having to provide and maintain such dry nitrogen environment is cumbersome.
Thus, the transport of titanium or zirconium alkoxide or aryloxide in the solid state requires the titanium or zirconium alkoxide or aryloxide to be solidified after its production. This is usually accomplished by allowing the titanium or zirconium alkoxide or aryloxide to form a solid and by forming it into suitable particles, which can then be bagged and transported as solid material. Generally, such solidification and particle formation require large and complicated equipment. Alternatively, titanium or zirconium alkoxide or aryloxide can be stored and solidified in special vessels after its production and then re-melted at the time of use. However, this also requires complicated equipment, like special vessels which either have self-heating capability or can be heated up in for example a hot room.
Such large and complicated equipment unnecessarily increases the capital investment, and is expensive and energy consuming to operate. Therefore, it is generally an object not to have to store and transport titanium or zirconium alkoxide or aryloxide in the solid state.
In view of the above, there is a need to decrease the holding temperature of titanium or zirconium alkoxide or aryloxide, such that titanium or zirconium alkoxide or aryloxide can be held as a liquid at a relatively low temperature. In that way, titanium or zirconium alkoxide or aryloxide catalyst could be advantageously transported and stored in liquid form at a relatively low temperature without having the above-mentioned drawbacks.
One solution to achieve this is to blend different titanium or zirconium alkoxides or aryloxides. For example, in case the desired catalyst to be used is Ti(OEt)4, this may be blended with for example 3-20 wt. % of Ti(OiPr)4 such that the resulting blend is a liquid in the standard state (at 25° C. and 100 kPa). Such liquid blends are commercially available. For example, Santa Cruz Biotechnology, Inc. markets sc-251257, a product that contains 80-97% of titanium(IV) ethoxide and 3-20% of titanium(IV) isopropoxide. Further, Sigma-Aldrich markets Aldrich-244759, a technical grade titanium ethoxide product that contains about 80% titanium(IV) ethoxide and about 20% titanium(IV) isopropoxide. However, said blends may still solidify if the temperature is reduced from the standard state temperature of 25° C. to a relatively low temperature, which may be as low as −10° C. or even lower, which temperature reduction could occur during transport and/or storage of these blends.
Further, a drawback of such blending, in a case where the catalyst is to be used in a process wherein aromatic carbonates are produced from a dialkyl carbonate and an aryl alcohol, is that alcohol impurities would be introduced into the process. For example, in a case where diethyl carbonate is reacted with an aryl alcohol, first ethyl aryl carbonate and ethanol are formed. If in such case, a blend of Ti(OEt)4 with Ti(OiPr)4 is used as a catalyst, a certain amount of isopropanol would be formed through the following general ligand exchange reaction (R=ethyl or isopropyl):Ti(OR)4+aryl alcohol→Ti(aryloxide)4+4ROH
This isopropanol impurity would then mix with the larger amount of ethanol. The formation of such alcohol mixture is disadvantageous in case the ethanol is to be recycled for example to a preceding step wherein further diethyl carbonate is to be made using such recycled ethanol (e.g. by reacting that ethanol with ethylene carbonate resulting in diethyl carbonate and monoethylene glycol), in which case the isopropanol first needs to be separated from the alcohol mixture, which is cumbersome. In addition, the isopropanol formed may react with the carbonates fed to and/or formed in the aromatic carbonate production process to form corresponding carbonates which would also disadvantageously contaminate product and process streams and thus complicate purification.
Therefore, it is an object of the present invention to find a way wherein said problems associated with handling solid titanium or zirconium alkoxide or aryloxide as well as said problems associated with handling titanium or zirconium alkoxide or aryloxide at a relatively high temperature to maintain it in a liquid state, are avoided or decreased, without having to blend in any other titanium or zirconium alkoxide or aryloxide as discussed above.